Several materials systems are being developed to solve the looming problems associated with energy generation, transmission, conversion, storage, and use. Superconductors are a unique system that provides a solution across a broad spectrum of energy problems. Superconductors enable high efficiencies in generators, power transmission cables, motors, transformers and energy storage. Further, superconductors transcend applications beyond energy to medicine, particle physics, communications, and transportation.
Superconducting tapes are becoming more and more popular. This is in part due to successful fabrication techniques that create epitaxial, single-crystal-like thin films on polycrystalline substrates (Y. Iijima, et al., “Biaxially Aligned YBa2Cu3O7-x Thin Film Tapes,” Physica C 185, 1959 (1991); X. D. Wu, et al., “Properties of YBa2Cu3O7 Thick Films on Flexible Buffered Metallic Substrates,” Appl. Phys. Lett. 67, 2397 (1995); A. Goyal, et al., Appl. Phys. Lett. 69, 1795 (1996); V. Selvamanickam et al., “High Performance 2G wires: From R&D to Pilot-scale Manufacturing,” IEEE Trans. Appl. Supercond. 19, 3225 (2009)). Superconducting films that are processed by this technique exhibit critical current densities comparable to that achieved in epitaxial films grown on single crystal substrates. Using this technique, several institutions have demonstrated pilot-scale manufacturing of superconducting composite tapes. One popular process used to manufacture superconducting tapes is called metal organic chemical vapor deposition (MOCVD) (V. Selvamanickam et al., “High Performance 2G wires: From R&D to Pilot-scale Manufacturing,” IEEE Trans. Appl. Supercond. 19, 3225 (2009)).
Current MOCVD methods and systems used for manufacturing of superconductor tapes have significant drawbacks, which are primarily rooted in design flaws (V. Selvamanickam et al., “Method for Manufacturing High-Temperature Superconducting Conductors,” U.S. Pat. No. 8,268,386). For example, FIG. 1 illustrates a schematic of the showerhead 100 used in current MOCVD systems used for manufacturing of superconductor tapes. The core of the MOCVD system is a reactor 110 which consists of a showerhead 115 to disperse the precursor flow 120 on a tape 125 and a heater 130 to heat the tape 125 by contact heating as it travels along the heater 130.
One major drawback of the current MOCVD design is that the heating and deposition mechanisms do not provide uniform heating or uniform deposition on the tape. These design flaws produce superconductor tapes with a poor surface microstructure, which can significantly deteriorate the tape's superconducting quality. For example, the superconductor tape is heated by a fairly bulky heater by means of contact heating, and so a constant heater temperature does not necessarily yield a constant tape temperature, especially in thicker films. In addition, since the tape travels quickly over the heater, there are sporadic losses of contact between the tape and heater. And because the tape has a very small mass, even brief losses of contact result in significant decreases in tape temperature. Furthermore, precursor flow is directed downwards towards the tape, and once it hits the tape it flows sideways across the tape. This non-uniform flow causes temperature differences across the tape, and thus deviations from the optimum temperature window. All of these temperature fluctuations can cause the process to deviate from the optimum temperature window and produce misoriented grain growth in the tape's surface microstructure. In addition, there are no solutions available in current systems to directly monitor tape temperature since there is no line of sight available from outside the reactor, and there is no room in the reactor to monitor the tape temperature directly without interfering with the precursor flow. Current MOCVD designs monitor temperature using a thermocouple inside the heating block. But because the tape temperature is not uniform deviations from the optimum temperature window typically go unnoticed.
Furthermore, the deposition flow from the showerhead reaches the tape only at the center, and most of the other flow is pumped out without fully reaching the substrate. This non-uniform precursor flow results in non-uniform superconductor growth, including misoriented grain growth. This deposition phenomenon is illustrated in FIG. 2, which is a finite element analysis of turbulent fluid flow and solid/fluid heat transfer in a current MOCVD system. The streamlines 200 show the flow path of the precursor, while the color differences show flow inhomogeneity. The finite element plot illustrates that a substantial fraction of the precursor (especially that injected away from the center of the showerhead) does not make it to the tape surface, which reduces the conversion efficiency of precursor to film. Also, the non-uniform flow rate will cause non-uniform film deposition rate, which in turn can cause inhomogeneities in the film.
The aforementioned heating and deposition drawbacks are exacerbated as the superconducting film is thickened during fabrication. Thus, as the film is thickened misoriented grain growth increases. For example, FIG. 3A shows the surface microstructure of a 1 μm thick superconductor tape 300 fabricated by current MOCVD methods. The microstructure is fairly homogenous with relatively little grain misorientation 310. However, as seen in FIG. 3B, a 2 μm thick superconductor tape 320 presents with a substantial amount of misoriented grains 330. FIG. 3C shows a cross section of the 2 μm tape 320 with a-axis grains 330. These illustrations prove that the misoriented grains predominantly form after the initial 1 μm of tape is fabricated.
As misoriented grain growth increases with tape thickness, critical current density (critical current/cross sectional area) decreases, i.e. the quality of the superconductor film degrades with increasing tape thickness. This phenomenon is illustrated in FIG. 4, which shows critical current density as a function of superconductor tape thickness in tape made by a current MOCVD system. One explanation for this trend is that increases in misoriented grain growth in thicker films impede the flow of current, thus resulting in lower current densities. And when the current density decreases, the quality of the superconducting tape degrades. This indicates that the present-day MOCVD process is not suitable to fabricate high-quality superconducting tapes thicker than approximately 1 μm.
Another drawback to current MOCVD design is that the process is inefficient and costly. For example, as explained above, the MOCVD's deposition design can result in non-uniform tapes where most of the flow is pumped out without fully reaching the substrate. In addition, the showerhead is positioned at a substantial distance from the tape and heating block in order to avoid premature precursor decomposition from the heating block. As a result, expensive precursors are wasted in the manufacturing process leading to lower throughput. In addition, the deposition process is relatively slow and results in low yields. For example, the current process uses 2,2,6,6-tetramethyl-3,5-heptanedionate (-thd) metal organic complexes of Y or other rare earth (RE), Ba, and Cu as a precursor. The -thd complexes are deposited on the substrate and dissociate at an appropriate temperature and oxygen partial pressure to form YBa2Cu3Ox superconductor (or REBa2Cu3Ox). In the present MOCVD system, precursor dissociation is entirely thermally activated by the high substrate temperature. But thermal activation alone does not result in complete dissociation of precursors, and so, precursor to film conversion efficiency is low—only about 15% of the theoretical value.
Furthermore, it would be desirable to activate precursors using plasma activation. However, it is not feasible to introduce plasma in prior art reactors because the metallic showerhead and susceptor don't allow for it (V. Selvamanickam et al., “Ultraviolet (UV) and plasma assisted metalorganic chemical vapor deposition (MOCVD) system,” U.S. Pat. Pub. No. 2004/0247779).
Thus, there is need in the art for methods and systems that can fabricate superconducting tapes having films with varied thicknesses at constant temperature, with uniform precursor deposition, and in high yields and efficiencies (e.g., >15%). There is also need in the art for methods and systems that can fabricate superconducting tapes with films at higher thicknesses (e.g., up to 3 μm thick) with minimal misoriented a-axis grain growth and high critical current densities. Finally, there is need in the art for a system that can fabricate superconductor tapes via plasma activation.